1. Field of the Invention
The present invention relates to a lithographic apparatus, device manufacturing method, and a measurement system.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist).
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Known lithographic apparatus comprise moveable components and objects, such as a substrate holder or table, which holds the substrate during exposure or a reticle stage. In many cases, the displacements of such moveable objects relative to a base have to be measured. Conventionally, interferometers and/or capacitive sensors are used for monitoring the displacement of the movable objects. Both systems have their disadvantages.
Interferometers are used for their accuracy, but are very expensive equipment. They also require the presence of high quality mirrors that have to be arranged very accurately onto the moveable object. The mirrors also have to be arranged under various angles, which makes the moveable object even more complex to produce. Also, interferometers require an extreme stable measurement environment with regard to composition, temperature, pressure, and turbulence.
Capacitive sensors, on the other hand, have a small range over which they can used, and their accuracy is not always sufficient. Moreover, they require a small distance between the sensor and the object to which the distance has to be measured.